A.R. Lakshmanan

A comparison of thermal neutron and gamma ray sensitivities of common TLD materials.

The thermal neutron responses in the range 1010 to 2*1012 n cm-2 and the gamma -ray sensitivities of common thermoluminescent materials (CaSO4:Dy, CaSO4:Tm, CaF2:Dy, CaF2:Mn, Li2B4O7:Mn, LiF (TLD-700, TLD-100, TLD-600), Mg2SiO4:Tb and BeO) have been studied in detail and the results compared with those published elsewhere.

Luminescence in CaSO4 : Dy phosphor - dependence on grain agglomeration, sintering temperature, sieving and washing

In the recently developed high-sensitive CaSO4 : Dy phosphor, sieving before the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermostimulated luminescence (TSL) sensitivity to γ-rays. The reduction in TSL sensitivity of higher sized grains observed earlier following the procedure of sieving after sintering has also more or less vanished. Maximum TSL sensitivity is seen after sintering around 700°C, whereas maximum photoluminescent (PL) sensitivity is seen after sintering around 325°C. While the observed increase in TSL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy3+ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy3+ environment on the grain surface perhaps due to oxygen incorporation. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO4 : Dy slightly; however, it does not influence TSL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TSL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature.

Gamma radiation induced sensitization in CaSO4:Dy TLD phosphor

Abstract High pre gamma exposures (> 100 R) and low post-annealing temperatures ( 600 °C) both sensitization and RTL were found to disappear. A model is postulated to explain the observed phenomena on the basis of competition between the sensitization traps and the RTL traps, in trapping the charge carriers generated during the test exposure. As a consequence of the appearance of new TL glow peaks in the temperature region of 200 to 300 °C, virgin phosphor was found to exhibit supralinear response above 100 R test-exposure. The sensitized phosphor was however found to show extended linearity up to 3 × 103 R. The dependence of the TL sensitivity of virgin CaSO4 :Dy on the annealing temperature in the range 300 to 700 °C, and its memory effect for gamma exposures in the range 500 to 107 R after a high temperature post-annealing treatment were also studied.

A New High Sensitive CaSO4 : Dy Thermostimulated Luminescence Phosphor

This paper reports the development of a high sensitive CaSO 4 : Dy thermostimulated luminescence (TLD) phosphor prepared by a new (N) inexpensive technique. Scanning electron microscope photographs reveal that its crystal morphology is quite different from the presently used CaSO 4 : Dy (P). The TSL sensitivity of N (<75 μm) is nearly 60% higher than that of P at low dose levels. Below 1 Gy, the major TSL glow peak of N as well as P occurs at the same temperature (∼230 °C). The 130 °C peak as well as the peaks in the region 300-350 °C are very small in N when compared to those in P. While the glow curve shape of N is nearly invariant with radiation dose up to 10 4 Gy, P exhibits drastic glow curve changes at dose levels ≥ 10 Gy. As a result, the supralinearity of N above 10 Gy is considerably less than that of P. ESR signal in N due to the SO 4 - and SO 3 - radicals related to peak(s) around 130 °C are less intense than those in P. Though the different X-ray diffraction lines of the two powder samples occur at the same d spacings, the relative intensities of almost all the minor lines are considerably reduced in N. The TSL and photoluminescent (PL) emission spectra show the characteristic 480 and 570 nm Dy 3+ lines in both samples but minor changes in their relative intensities between them were noticed. Since the new CaSO 4 : Dy material has better basic TSL characteristics, the new preparation technique (N) is advantageous relative to the presently (P) used one.

Gamma radiation induced sensitization and photo-transfer in Mg2SiO4: Tb TLD phosphor

Abstract Mg 2 SiO 4 : Tb TLD phosphor was found to show enhanced TL sensitivity to both gamma and UV radiations after high pre-gamma exposures (>100 R ) and a post-annealing treatment at 300°C for 1 h. Maximum sensitization factors of 2.8 and 55 were obtained at the pre-exposure levels of 5.2×10 1 C/kg and 1.3×10 3 C/kg for gamma and UV test radiations respectively. The near constancy of the intensity of the residual TL (RTL) peak at 500°C for the sensitized sample with increasing test-gamma exposures has ruled out the re-trapping model proposed earlier for the gamma radiation induced sensitization in this phosphor. The T max for the sensitized phosphor was found to occur at a higher temperature compared to that for the virgin phosphor. The dependence of sensitization on RTL was explained qualitatively on the basis of competition between sensitization traps (having higher energy than the dosimetry traps) and RTL traps while capturing the charge carriers generated during the test-gamma exposure. The sensitization observed in this phosphor to UV test radiation was found to be a consequence of the photo-transfer of charge carriers from deep (RTL) traps to the shallow (dosimetry) traps. The reduction in RTL peak (500°C) intensity of the sensitized sample with increasing test-UV exposure has demonstrated the photo-transfer mechanism in this phosphor. The TL response of the virgin Mg 2 SiO 4 : Tb phosphor was found to be supralinear to both gamma and UV radiations. The TL response of the sensitized phosphor was found to be linear to gamma radiation and sublinear to UV radiation.

Mixed field dosimetry with CaSO4:Dy

The thermal neutron response and gamma -ray sensitivity of CaSO4:Dy as a function of Dy concentration were studied in detail. CaSO4 with 0.2% Dy has maximum gamma -ray sensitivity. The thermal neutron response of CaSO4:Dy increases rapidly with concentration beyond 0.1% Dy. CaSO4:0.05% Dy (or Tm) has high gamma -ray sensitivity coupled with low thermal neutron response and hence is useful for gamma -ray dosimetry in mixed fields. A mixture of LiF TLD-100 and TLD-700 was arrived at for rem equivalent dosimetric purposes in a mixed field consisting of thermal and intermediate energy neutrons and gamma -rays. Mixtures of CaSO4:Dy with various lithium based materials were also selected for the same purpose. The thermal neutron response and glow peak temperature of CaSO4:(Dy, 6Li) were studied as a function of 6Li concentration.

Mechanism of nonlinearity in the response characteristics of thermoluminescent dosimeters

A new model is proposed for explaining the nonlinearity in the response characteristics of three common thermoluminescent phosphors (CaSO4:Dy, CaF2:Dy and Mg2SiO4:Tb). The increased trapping of charge carriers in the dosimetry traps during irradiation, due to a reduction in trapping efficiency of deep traps caused by radiation damage, was found responsible for the supralinearity in TL response of dosimetry peaks and sublinearity in TL response of high temperature peaks of these phosphors at high gamma ray doses. The dose versus TL response characteristics of deep traps in CaSO4:Dy was found to vary drastically from batch to batch, and in CaSO4:Dy, the sensitisation observed after a gamma ray dose of 103 Gy and annealing at 400 degrees C was found useful in low dose measurements.

Ultraviolet response of CaSO4 : Dy

A simple method for measuring ultraviolet exposure using the thermoluminescent properties of CaSO4:Dy is presented in this paper. In general, three peaks at 145, 215 and 365 degrees C were observed by ultraviolet irradiation of virgin phosphor. The peak at 365 degrees C was found to be very small and that at 145 degrees C fades away within a week; no appreciable fading was observed for the 215 degrees C peak. Further, the peak at 215 degrees C was found to coincide with the gamma dosimetric peak and this peak was chosen for ultraviolet exposure measurements. A standard quartz pen-ray lamp was used for calibrating the phosphor both by peak height measurement and by integration. The dosimetric peak has been found to be linear from the detection threshold of 400 erg mm-2 to 4 X 10(4) erg mm-2. Beyond this exposure the phosphor behaved supralinearly to ultraviolet exposure up to the studied range of 5 X 10(5) erg mm-2. The phosphor exhibited strong dependence on photon energy. The TL sensitivity increases sharply by a factor of 300 as the wavelength is decreased from 250 to 200 nm. The response is negligibly small above 350 nm. Other important characteristics such as particle size dependence, fading characteristics, thickness dependence and re-usability are also presented. The ultraviolet sensitivity increased by a factor of 3 when the grain size range was changed from 105-210 mum. Self-shielding by a factor of 2 was observed when the sample thickness was increased from 10 to 72 mg cm-2.

Luminescence spectra of CaSO4 with Ce, Dy, Mn and Ag codopants

Thermoluminescence (TL) and radio-thermoluminescence spectral analysis techniques have been applied to doped calcium sulphate samples designed for radiation measurements at elevated temperatures. CaSO4 :Dy, when co-doped with Ag, provides a TL dosimetric peak near 350 °C which is useful for radiation measurements at high temperatures. Dopants of Ce, Mn and Dy variously move the peak temperature from 400 °C to 200 °C. Each dopant ion gives a characteristic emission spectra, which for CaSO4 :Ce, Mn samples indicate that there is a systematic temperature difference of ~7 °C between the glow peaks from the Ce and Mn sites. The CaSO4 :Dy samples show a discontinuity in the emission wavelength from the Dy ions near T = 200 °C and a decrease in the radioluminescence fluorescence in the same temperature region. In each case it is proposed that the dopants form part of large, complex defects, instead of isolated trapping and recombination centres. The data offer further evidence for a localized phase transition of the defect complex at 200 °C. Low-temperature data, from 20 K, show similar differences in the peak temperature from the various dopants and additionally indicate reproducible discontinuities in the wavelength positions and intensities, for all samples, at T = 230 K. This again suggests structural phase adjustments of the defect sites.

Thermoluminescence studies in LiF dosimeter irradiated at 77K and their implications on theoretical models

In LiF:Mg, Ti dosimeters, gamma -irradiation at low temperature (77K) reduces the TL intensities of peaks 7 (260 degrees C) and 10 (450 degrees C) drastically whereas the TL intensity of peak 5 (200 degrees C) remains nearly the same as that after irradiation at room temperature (RT). This implies that during irradiation, interstitial migration is involved in the formation of traps giving rise to peaks 7 and 10. The generic nature of peaks 7 (Z2) and 10 (Z3) traps and their (Z2 to or from Z3) trap conversion are further demonstrated. Since diffusion processes are inhibited, F and Z centres are not formed efficiently in LiF dosimeters irradiated at 77K. Studies on supralinearity, sensitisation, thermal history, optical absorption, optical bleaching, phototransfer and LET dependence following irradiation at RT and at 77K imply that four different processes (competing nonluminescent centre, track interaction during read-out, Z centre and mobile interstitial) are operating simultaneously to varying extents during the trapping/luminescent stage of different TL glow peaks. The photon emission mechanisms of these peaks are discussed. The claim that supralinearity of peak 5 is related to Z3 production is disproved. Further studies on Z centre identification in LiF:Mg, Ti are indicated.